1. Field of the Invention
This invention relates to a process for hydrotreating a heavy oil containing soluble metals (to be referred to simply as "metals" hereinbelow) such as organometallic compounds. More specifically, this invention pertains to a novel two-step hydrotreating process for a heavy oil which comprises catalytically hydrotreating the heavy oil in two steps using a first-step catalyst zone comprising a catalyst having higher desulfurizing activity than demetallizing activity and a second-step catalyst zone having higher demetallizing activity than desulfurizing activity.
2. Description of the Prior Art
Heavy oils, especially residual oils from distillation of crude oils at an atmospheric or reduced pressure, contain in concentrated form almost all of metals, asphaltenes and residual carbon precursor substances which were present in the crude oil, and also have sulfur and nitrogen in high concentrations. Thus, these heavy oils have only limited applications. It is known that among the various hetero elements contained in heavy oils, metals constitute permanent poisoning substances on catalysts in the catalytic treatment of the heavy oils. Many methods have therefore been proposed in the past to remove these metals. These conventional hydrotreating methods, known generally as hydrodesulfurizing or hydrodemetallizing methods, are superior methods which can afford treated oils having low contents of metals, asphaltenes, sulfur and nitrogen in high yields. For use in these treating methods, there have been developed hydrodesulfurization catalysts supported mainly on an alumina or silica-alumina carrier and having sufficiently high desulfurizing activity and a long catalyst lifetime. These catalysts, however, are not always suitable for concurrent application to the demetallizing, or metal removal, of feed oils having a high content of metals. For example, when a hydrodesulfurized oil is used as a starting material for catalytic cracking, it is necessary to reduce the metal content of the stock to not more than 10 ppm, preferably to several ppm, beforehand in order to avoid degradation of cracking catalysts. Although it is known that such thorough demetallization is technically possible by performing thorough desulfurization under severe reaction conditions, such a thorough desulfurizing-demetallizing treatment with a desulfurization catalyst is undesirable because the amount of hydrogen chemically consumed increases markedly with an increase in the degree of desulfurization. It is noted in hydrodesulfurization of ordinary residual oils that the amount of hydrogen consumed chemically per unit amount of sulfur removed increases gradually at a desulfurization rate of 60 to 70% or more, and strikingly at a desulfurization rate of more than 80%, especially more than 90%. On the other hand, the sulfur content of the catalytic cracking stock is preferably low in order to reduce the amount of sulfur oxide in exhaust gases from a catalyst regenerating tower, but is not particularly limited for the purpose of obtaining light oils in high yields. The light oils produced in the catalytic cracking process can be easily hydrodesulfurized under mild reaction conditions with a small amount of hydrogen chemically consumed. Accordingly, when it is desired to obtain materials for catalytic cracking, etc. from heavy oils having large amounts of soluble metals, thorough demetallization, rather than desulfurization, of the heavy oils is required, and to prevent an increase in the amount of hydrogen consumed chemically in this case, it is rather preferred to decrease the rate of desulfurization. Another imperfection of the desulfurizing-demetallizing method using hydrodesulfurization catalysts is that these catalysts decrease in activity as the metals in the feedstock deposit thereon, and with it, the properties of the product oils, characterized by their sulfur and metal contents, vary continuously. In order to use the treated oils continuously as feedstocks for catalytic cracking, their properties are preferably maintained constant. Variations of the properties of the feedstocks are extremely undesirable because they result in variations in the operating conditions of the catalytic cracking process for these treated oils fed continuously and also in the properties, yields, etc. of the cracked products.
For use in the so-called hydrodemetallizing method, catalysts having a very long catalyst lifetime, such as sepiolite-type demetallizing catalysts, have been suggested. Methods utilizing these catalysts prove superior in demetallization of heavy oils because the use of these catalysts leads to a reduced amount of hydrogen chemically consumed. However, even the use of these long-life demetallizing catalysts causes gradual changing of the properties of the treated oils as the catalysts undergo degradation, although it is not as abrupt as is the case with the desulfurization catalysts. Furthermore, in thorough demetallization with demetallization catalysts, it is noted that a considerable amount of sulfur is also removed and an excessive amount of hydrogen chemically consumed is necessary. However, the degree of desulfurization cannot be kept at a desired level depending upon the properties of the feedstock oil. For this reason, the hydrodemetallizing method using demetallization catalysts which mainly induce demetallization is not entirely suitable for the demetallizing-desulfurizing treatment of heavy oils.
The present inventor already disclosed in Japanese Laid-Open Patent Publication No. 98308/1978 a so-called demetallizing-desulfurizing process characterized by using a combination of a desulfurization catalyst and a demetallization catalyst having specified properties. This process is based on the surprising experimental fact that while a direct desulfurizing catalyst having a large average pore diameter usually considered to be suitable for treatment of heavy oils is markedly susceptible to degradation in the hydrodesulfurization of demetallized oils, contrary to expectation from the conventional common knowledge, a catalyst for desulfurizing distillated oils which has a small average pore diameter rather has high activity and a long catalyst lifetime in the hydrodesulfurization of demetallized oils. This process is also based on the discovery that the content of metals in light fractions is especially decreased. It has also been noted that in the hydrodesulfurizing treatment of demetallized oils, the rate of demetallization is much lower than the rate of desulfurization, and fairly large amounts of metals remain in the heavy fractions even after the two-step treatment. In order, therefore, to markedly reduce the total metal level of the treated oil and also to reduce the sulfur level to a desired point, it is necessary to perform thorough demetallization in the hydrodemetallizing step and then to further desulfurize the demetallized oil. Such a thorough demetallizing treatment with hydrodemetallizing catalysts requires very severe reaction conditions as is the case with the thorough demetallizing treatment with hydrodesulfurization catalysts.
The inventor also disclosed in Japanese Laid-Open Patent Publication No. 90503/1977 a demetallizing-desulfurizing process in which at least a part of hydrogen sulfide formed in the step of desulfurizing a demetallized oil is recycled to the demetallizing step. This process is based on the discovery of the phenomenon that the activity of a demetallizing catalyst, contrary to the conventional common knowledge, contributes greatly to the increase of the partial pressure of hydrogen sulfide.
The present inventor made extensive investigations in order to apply these prior findings mainly to the production of product oils having a very low content of metals. These investigations finally led to the present invention.